This invention relates to circuit packs which include a microprocessor, and, in particular, to a circuit for applying a reset pulse to the microprocessor.
Generally, when circuit packs are plugged into a backplane, and at other times, a reset pulse must be applied to the microprocessors included in the pack. For example, a Transmit-Receive Unit (TRU) circuit pack, which is part of a Remote Terminal (RT) in a digital loop carder transmission system, typically includes two microprocessors which must be reset.
In a typical prior art approach, a power-up reset circuit would be :. triggered by the ramped supply voltage applied to the pack, and this pulse would reset the microprocessors. Such reset circuits typically require a ramp of 0 to 4 volts in less than 100 microseconds to be properly triggered. However, in Subscriber Loop Carder (SLC.RTM.) systems, the ramp can be 20 to 30 milliseconds. Such a slow ramp has caused the power-up reset to trigger at too low a voltage to reset the microprocessors.
It is, therefore, an object of the invention to provide a reset circuit for a microprocessor which ensures a reset in the event of a slow ramp-up in voltage of the power supplied to the circuit.